Various types of thermal treatment apparatus are used in the fabrication of semiconductor devices, to perform processes such as oxidation, diffusion, chemical vapor deposition (CVD), or annealing on a semiconductor wafer that is the substrate to be processed. One of these is a vertical thermal treatment apparatus which is capable of batch-processing a large number of wafers simultaneously and which is provided with a reaction tube (process tube) that functions as a thermal treatment chamber within a type of furnace called a hot-wall furnace, which has a heater disposed around the periphery thereof. A large number of substrates to be processed are held horizontally and in multiple stages at a suitable spacing in the vertical direction by a wafer boat that is means for supporting substrates to be processed within the reaction tube, and this large number of wafers is subjected to a thermal treatment simultaneously.
In the thus configured prior-art thermal treatment apparatus, thermal radiation from the heater is more likely to be incident from more directions on the peripheral edge portions of the wafers held in the wafer boat than on the central portions thereof. A dramatic difference can easily occur in the temperature response characteristics of the central portions and peripheral edge portions of the wafers, particularly if the temperature increases or decreases rapidly, and this tends to cause slipping (crystal distortion) in the wafers, due to internal stresses generated by temperature differences within the surfaces of the wafers. To solve this problem, the wafer boat in the prior-art thermal treatment apparatus is provided with ring-shaped support plates to support the under-surfaces of peripheral edge portions of the wafers, in surface contact therewith (a wafer boat provided with such ring-shaped support plates is called a ring boat), with the objective of using the thermal capacity of these support plates to control the temperature response characteristics of the peripheral edge portions of the wafers, and thus reduce the temperature difference within the surface between the central portion and the peripheral edge portion of each wafer.
However, heat conduction between each wafer and its ring-shaped support plate in the above described prior-art thermal treatment apparatus is not by radiation alone; heat conduction through the gases present between the wafer and the support plate (heat conduction through gases, also called convective heat transfer) is also an important element. For that reason, if the temperature within the reaction tube is raised or lowered rapidly in a reduced-pressure state, the effects of the ring-shaped support plates are not fully realized and it becomes difficult to sufficiently reduce the temperature difference between the central and peripheral edge portions of each wafer. Therefore, since it is difficult to raise or lower the temperature rapidly under a reduced pressure in this prior-art thermal treatment apparatus, there is a limit to improvements in throughput.
The object of the present invention is therefore to provide a thermal treatment apparatus that is designed to improve throughput by reducing the difference in temperature within the surface between the central portion and peripheral edge portion of each wafer, not only under normal pressure within a thermal treatment chamber but also under reduced pressures, in order to solve the above problem in the prior art.